1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. More particularly, the present invention relates to a structure of a thin film transistor (TFT) and a method for fabricating the same.
2. Description of the Related Art
A thin film transistor (TFT) typically includes a gate, a gate dielectric layer, a channel layer, a drain and a source, and is usually used as a switching device in a semiconductor apparatus. Since a TFT has a gate, a source and a drain, its operation is the same as that of a conventional MOS-FET. The TFT family includes amorphous silicon(□-Si) TFT, which is formed by sequentially forming a gate, a gate di-electric layer, an □-Si channel layer and a source/drain on a substrate. The gate is constituted of a single metal layer of aluminum (Al), chromium (Cr), tungsten (W), tantalum (Ta) or titanium (Ti), or constituted of multiple metal layers.
However, when a single layer of aluminum or aluminum alloy is used to form the gate, the surface of the layer easily reacts with oxygen in the atmosphere to form aluminum oxide that cannot be effectively etched in the subsequent etching process. Therefore, a protective layer, such as a nitride layer, is usually formed on an aluminum or Al-alloy gate to prevent formation of aluminum oxide. However, such a method causes etching residues to form around the top of the gate during the etching process, resulting in an eave effect that will affect subsequent processes.
On the other hand, when the gate is constituted of multiple metal layers including aluminum or aluminum alloy and other metals, such as chromium (Cr) or molybdenum (Mo), the underlying aluminum or Al-alloy layer having a higher etching rate is etched faster to produce undercuts in the etching process. Furthermore, etching residues are also formed during the etching process of the multiple metal layers, so that an eaves effect is caused to raise the contact resistance and the wire resistance of the gate and lower the performance of the device. In addition, the process for forming multiple metal layers is surely more complicated than that for forming a single metal layer. Moreover, the above problems also occur in the process for forming the source/drain.